Control system for extraction and injection of steam from and into a turbine

ABSTRACT

The present invention relates to a control system for controlling injection and extraction of steam in a steam turbine (1), the same valves (5,6) being used both for injection of steam to the turbine (1) and for extraction of steam from the turbine (1) . The valves (5, 6) commonly used for injection and extraction are controlled to be timely opened and closed in dependence on the pressure conditions at injection points (2,3) and extraction points (2,3), respectively, in the turbine and on the pressure (P3) prevailing in a process network connected to the valves (5, 6) such that injected or extracted steam is utilized in an optimal manner.

TECHNICAL FIELD

The present invention relates to a control system for control of steamflows when injecting and extracting steam in connection with steamturbines, especially when using steam turbines in industrial processeswhere the steam consumption of the turbine varies.

BACKGROUND OF THE INVENTION

In certain applications industrial steam turbines are connected to aprocess network with greatly varying steam consumption. An optimalutilization of the turbine then receives that at certain times steam isto be injected into the turbine from the process network, whereas onother occasions steam is to be extracted from the turbine to the processnetwork.

In order to inject and extract steam at an optimal location in theturbine, multi-point injection/multi-point extraction in accordance withknown methods for injection/extraction of steam within the turbinetechnique are utilized. In the case of injection, the optimal point isas high up in the turbine as possible, that is, it takes place at apoint in the turbine with high steam pressure. Optimal extraction iscarried out in a corresponding manner as far down in the turbine aspossible, that is, at a point with low pressure in the turbine.

According to the conventional technique for externally controlledextraction or injection, separate valves are currently used forcontrolling the steam flows for injecting steam into the turbine, thatis, a number of valves equal to the number of injection points arecontrolled to open and close to control any steam flows to the differentinjection points. In a corresponding way, any steam flows from theturbine through extraction points in the turbine are controlled by meansof separate valves in connection with extraction of steam from theturbine. The number of extraction valves is then equal to the number ofsteam extraction points from the turbine.

Consequently, with the current technique for controlling injection andextraction of steam into and from a steam turbine, it is necessary toprovide a number of valves equal to the sum of the number of injectionand extraction points in the turbine. This entails a large cost for thevalves and an unnecessarily complicated valve control as well as, incertain cases, disadvantages from the performance point view of.

SUMMARY OF THE INVENTION

The present invention relates to a control system for control ofinjection and extraction of steam in a steam turbine in which, the samevalves being used both for injection of steam into the turbine andextraction of steam from the turbine. The valves which are common forinjection and extraction are controlled to be timely opened and closedin dependence on pressure conditions at injection points and extractionpoints, respectively, in the turbine and on the pressure prevailing inthe process network connected to the valves such that injected orextracted steam is utilized in an optimal manner.

The conduits for extraction of steam are the same as the conduits forinjection of steam.

In the following description, a valve is designated and considered alower valve if it communicates with and controls the steam from and toan extraction/injection point, respectively, which is located at aconnection point of the turbine at a lower pressure level in relation tothe other extraction/injection points. Correspondingly, a valve isdesignated a higher valve if it communicates with and controls the steamflow from an extraction/injection point which is located at a connectionpoint of the turbine at a higher pressure level in relation to the otherextraction/injection points.

The present invention operates according to the principle that valvesduring, for example, extraction of steam are opened in,a certainsequence. Primarily, a lowest valve is opened which extracts steam fromthat of the two extraction conduits which is in communication with theextraction point with the lowest steam pressure in the turbine, referredto here as the lowest extraction point. If, at some time, the pressureat the lowest extraction point does not maintain a value which isnecessary for the connected process network, which is sensed by adifferential pressure gauge across the valve, this lowest valve isclosed, whereas a higher valve which attends to the extraction of steamfrom a higher extraction point located at a point in the turbine with aprevailing higher steam pressure is opened and extracts steam to theprocess network. If the pressure level at lower extraction points shouldagain rise, the lower valve is opened again, the higher valve thus beingclosed. This would produce the desired effect, that is, that steam withthe lowest pressure would be extracted.

If extraction from more than two extraction points is controlled in away according to the present invention, valves for extraction of steamare opened sequentially towards the extraction points with increasingsteam pressure. However, a return to more optimal extraction isconstantly made if the pressure at an extraction point with lower steampressure again exhibits a value exceeding the pressure in the connectedprocess conduit, whereby, sequentially, valves controlling the steamflow in extraction conduits are opened stepwise with increasingly lowerpressure until the extraction conduit with the lowest steam pressurewhich exceeds the process pressure is open for extraction of steam fromthe turbine.

When injecting steam, the same valves as those mentioned above arecontrolled to be opened in a sequence which is opposite to the sequenceduring the extraction. This means that in case of two-point injection,steam from the process network, if the pressure of this steam issufficient, is primarily injected via the opened highest valve to thatof the two injection conduits which is connected to the higherpressurized part in the turbine, which is here referred to as thehighest injection point. If the pressure of the steam from the processnetwork at some time should be lower than the pressure at this highestinjection point in the turbine, this highest valve is closed whereas thelower valve is opened to make possible injection of steam from theprocess network at a point in the turbine with lower pressure, which isdone at a lower injection point in the turbine. If the pressure level ofthe steam in the process network should rise again, the injection at thehigher pressure will change to the highest injection point in theturbine provided that the steam pressure in the process network is atleast as great as the pressure at the highest injection point of theturbine.

If the injection of steam into more than two injection points iscontrolled in a way according to the invention, valves for injection ofsteam to the turbine are opened in sequence towards injection pointswith reduced steam pressure. However, a return to more optimal injectionis constantly made if the pressure at an injection point with highersteam pressure again exhibits a value lower than the pressure in theconnected process conduit, whereby, sequentially, valves controlling thesteam flow in injection conduits are opened stepwise with increasinglyhigher pressure until the injection conduit with the highest steampressure which is lower than the process pressure is open for injectionof steam to the turbine.

Electronic control equipment attends to opening and closing of the servovalves for achieving injection or extraction of steam to the optimalinjection or extraction points in the turbine in dependence on pressureconditions in a process network connected to the turbine and atinjection and extraction points.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE illustrates a diagram of a control system for two-pointextraction and injection of steam, respectively, in a steam turbineconnected to a process network.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, an embodiment of a control system according to theinvention for extraction and injection of steam in a turbine will bedescribed, in which extraction of steam from the turbine and injectionof steam to the turbine, respectively, are carried out, by describing,in the example, the function of the control system for two-pointextraction and injection.

The FIGURE illustrates a steam turbine 1, which symbolizes two or moreturbine stages with a lowest tapping point 2 and a highest tapping point3 for steam from and to the turbine 1, respectively. Consequently, thetapping points 2, 3 in the turbine 1 serve as both injection point andextraction point and will be referred to in the following according totheir current function.

Extraction and injection of steam at the tapping point 2 are performedby a lowest servo valve 5 with an associated servo 5a. In acorresponding way, extraction and injection of steam at the tappingpoint 3 are performed by a highest servo valve 6 with an associatedservo 6a. The tapping points 2, 3 of the turbine 1 for injection andextraction of steam are connected via the valves 5, 6 to a connectingconduit common to the two valves, so called process conduit 4, which inturn is connected to a process network. The process conduit 4 suppliessteam to the process network during extraction or removes steam from theprocess network during injection. The steam pressure P3 in the processnetwork is measured at the process conduit 4 by a pressure gauge 7.

The pressure difference between the steam pressure P3 in the processnetwork and the pressure P1 at the lowest tapping point 2, that is, thepressure across the lowest valve 5, is measured by means of adifferential pressure gauge 8, which supplies an output signal dP_(I)which indicates if P3>P1. In a corresponding way, the pressuredifference between the steam pressure P3 in the process network and thepressure at the highest tapping point 3, that is the pressure across thehighest valve 6, is measured with a differential pressure gauge 9, whichin turn supplies an output signal dP_(II) which indicates if P2<P3.

The output signal from the pressure gauge 7 which measures the steampressure in the process conduit 4 is supplied to a first control unit 10and to a second control unit 11. The first control unit 10 is activeduring extraction of steam from the turbine 1, whereas the secondcontrol unit 11 is active during injection of steam to the turbine 1.The two control units 10, 11 have a common set value generator SVG,which sets a reference value pressure level for the control units 10, 11through a dead-band unit DB. The dead-band unit DB provides a dead bandin the control system, which dead band defines a certain small pressureinterval within which activation of another control unit cannot takeplace to ensure for the control system a distinct switchover between thetwo control units 10, 11 in dependence on the pre-set reference valuepressure and the current pressure P3 in the process network foractivation of the correct control unit 10, 11 depending on whetherextraction or injection of steam is called for. The two control units10, 11 act on the valves 5, 6 through a maximum value selector MAX,which allows the greater of the two signals from the two control units10, 11 to be passed on to a split-range device 12. The split-rangedevice 12 operates according to the sequence a-b when the extractionunit 10 is activated, that is, extraction of steam from the turbine isto be carried out. This causes devices connected to the output a of thesplit-range device 12 to be controlled, in this case a servo positioncontrol unit 13 for the valve servo 5a for the lowest valve 5, beforedevices connected to the output b of the split-range device 12 receivecontrol signals from the split-range device 12. The split-range device12 operates in a corresponding way but in the sequence b-a when theinjection control unit 11 is activated, which means that devicesconnected to the output b, in the present example a servo positioncontrol unit 14 for the valve servo 6a for the highest valve 6, arecontrolled before devices connected to the output a receive controlsignals from the split-range device. Switching between the two sequencesof the split-range device 12 is carried out by a switching member 15,which senses which of the control units 10, 11, monitoring extractionand injection, respectively, is active. The switching member 15 has aSet-Reset function, which is controlled by an active control unit 10,11.

In extraction mode the lowest valve 5 and the highest valve 6 operatesequentially controlled, the lowest valve 5 being opened first, providedthat the pressure conditions are fulfilled, that is, that P3<P1. Thehighest valve 6 is opened only if the steam flow through the lowestvalve 5 is insufficient to maintain the required process pressure P3. Ifthe process pressure P3 is greater than P1, opening of the lowest valve5 is prevented by a member for forced closing comprising the switchesS1, S2 and the AND gate 20, which influence the valve servo 5a to closethe lowest valve via a MIN-value selector MIN1. The entire extractionsteam flow will then pass through the highest valve 6. The blocking orforced closing of the lowest valve 5 can only be activated when thecontrol unit 10 for extraction is active. If the lowest valve is blockedand the pressure difference across the lowest valve should change, sothat P3 again becomes smaller than P1, for example because of a changeof the stated output or caused by a changed process steam consumption,that is, a change of P3, sequential control will be resumedautomatically, which means that the lowest valve which is first in thesequence opened is again.

In injection mode the lowest valve 5 and the highest valve 6 operatesequentially controlled, the highest valve 6 being first opened and thelowest valve 5 being opened only if the steam flow through the highestvalve 6 is insufficient to maintain the required process pressure, thatis, if, for example, the highest valve is unable to swallow the requiredsteam flow. If the pressure on the turbine side of the highest valve 6should become too high to allow any injection at all, that is, if thepressure P2 becomes greater than P3, the highest valve 6 is blocked orforcibly closed via a blocking member comprising the switches S3, S4 aswell as the AND gate 25, which influence the valve servo 6a to close thehighest valve via a MIN-value selector MIN2, the entire steam flow thusbeing controlled to the turbine 1 via the valve 5 during injection. Theforced closing of the highest valve 6 can only be activated when thecontrol unit 11 for injection is active. If the highest valve 6 isblocked and the differential pressure across the highest valve shouldchange, caused, for example, by a change of the output, normalsequential control is automatically resumed, the highest valve 6 whichlies first in the control sequence during injection thus being openedagain.

During extraction the first member for forced closing can block thelowest valve 5, if necessary. The member comprises a switch S1, which iscontrolled by the signal dP_(I) from the differential pressure gauge 8.The control signal influences a contact in the switch S1, on the inputof which there is permanently a logical one. When the signal dP_(I)indicates that the pressure P3 is greater than P1, the contact in theswitch S1 is closed, whereby a logical one is fed via the output of theswitch to an AND gate 20. On the other input to the AND gate 20 there isa logical one if extraction is to be carried out. From this follows thatthe AND gate passes on a logical one via its output to a switch S2. Theswitch S2 receives this logical one from the AND gate 20 as a controlsignal, the contact in the switch S2 thus being closed, which means thatan analog zero bias is passed on via the contact in the switch S2 to theMIN-value selector MIN1. Since the MIN-value selector MIN1 senses thiszero bias as the lowest applied signal voltage, the valve servo 5a willclose the lowest valve 5 for extraction. If the pressure P3 in theprocess network is smaller than the extraction pressure P2, the switchS1 is not closed, or if extraction is not to be carried out, such that alogical one is not fed from the control unit 10 to the AND gate 20,consequently in both cases no forced closing of the lowest valve 5 takesplace, since zero bias is not present out from the switch S2. In thesecases, the position of the lowest valve 5 is determined by thesplit-range device 12.

In the same way as for the lowest valve 5, during injection the highestvalve 6 is controlled to forced closing if the injection pressure fromthe process network P3 is lower than the pressure P2 at the turbine sideof the highest valve.

The control system according to the description may be extended tocontrol extraction and injection of steam at three or more commontapping points in a turbine. In such an extended control system with,for example, three valves, there is instead used a split-range devicewith the sequence a-b-c during opening of the valves in connection withextraction and the sequence c-b-a in connection with injection. At thesame time, an additional forced closing device for the third valve isintroduced in accordance with the solution described above, where thedifferential pressure across the third valve via the third forcedclosing device determines whether this third valve has to be closed.

I claim:
 1. A control system for controlling extraction and injection of steam in a steam turbine with at least two extraction/injection points, said steam turbine being connected to a process network, said control system comprising: at least two valves, each of said valves being associated with an extraction/injection point, and control equipment for controlling operation of said valves for extraction and injection of steam into said steam turbine, wherein the valves which are used for extraction of steam from the steam turbine to the process network are the same as the valves which are used for injection of steam from the process network to the turbine.
 2. A control system according to claim 1, wherein the extraction of steam is effected by opening of a valve associated with an extraction point in the turbine with the lowest steam pressure, said lowest steam pressure exceeding a steam pressure of the process network; and wherein the injection of steam is effected by opening of a valve associated with an injection point in the turbine with the highest steam pressure, said highest steam pressure being lower than the steam pressure of the process network.
 3. A control system according to claim 2, further comprising a pressure gauge, a set value generator, and a first and second control unit and wherein in dependence on a comparison of said steam pressure in the process network sensed by said steam pressure gauge and a pre-set reference value in said set value generator it is determined whether extraction or injection is to be carried out, whereby in case of extraction said first control unit is activated and in case of injection said second control unit is activated.
 4. A control system according to claim 1, wherein said valves comprise at least a lower valve and a higher valve, said lower and higher valves being operated in a sequence during injection and extraction of steam and wherein during the extraction of steam said lower valve is opened first in the sequence and during the injection of steam said higher valve is opened first in the sequence.
 5. A control system according to claim 4, further comprising an active control unit connected to and controlling a split-range device, which determines an injection sequence or an extraction sequence for opening of the valves.
 6. A control system according to claim 5, further comprising a member including switches and an AND gate, which in case of simultaneous active extraction and a differential pressure signal dP_(I), from an active lower valve for extraction, showing an extraction pressure which is lower than the pressure P3 in the process network, forcibly closes said lower valve, whereby the extraction switches over to the nearest higher valve.
 7. A control system according to claim 5, further comprising a member including switches and an AND gate, which in case of simultaneous active injection and a differential pressure signal dP_(II), from an active higher valve for injection, showing an injection pressure exceeding the pressure P3 in the process network forcibly closes said higher valve, whereby the injection switches over to the nearest lower valve.
 8. A control system according to claim 6, further comprising another member including switches and an AND gate, which in case of simultaneous active injection and a differential pressure signal dP_(II), from an active higher valve for injection, showing an injection pressure exceeding the pressure P3 in the process network forcibly closes said higher valve, whereby the injection switches over to the nearest lower valve. 